how to debug "sql subquery returns more than 1 row" error

how to debug "sql subquery returns more than 1 row" error

We have a huge SQL script involving tens of tables, subqueries, hundreds
of attributes. It works perfectly in the test database but returns sql
subquery returns more than 1 row error when running in the production
database. The script was working perfectly up until now. The problem is,
all I get is a one-line error specified above with no clues whatsoever
which exact subquery causes the error which makes it near to impossible to
debug. The question is, how am I supposed to know which line of the SQL
causes the error? Is there any way to "debug" it line by line like you
would do it in a programming language? I am using TOAD with Oracle 11g.